Sludge production in the South West Water region is predicted to rise from 25,000 tonnes dry solids to 48,000 by the 2011, mainly as a result of the marine improvement programme and the cessation of disposal of sludge to sea by the end of 1998. Additional factors include population growth and enhanced discharge standards for inland wastewater treatment works.
Ernesettle stream with the main sludge storage tank showing the odour control unit to the left.
South West Water’s sludge strategy of 1992, revised in October 1996, for coping with the dramatic increase in sludge production contained three distinct elements:
essential improvements to existing digester facilities with regard to health and safety process, odour control, liquor sludge storage and sludge dewatering;
upgrade of sludge handling and disposal facilities; and
the provision of sludge thermal drying centres.
The Plymouth (Ernesettle) works was chosen as a sludge treatment and handling centre. The works is located to the north west of Plymouth and is one of four major treatment works serving the city of Plymouth. The Ernesettle sludge centre was also to handle and treat imported sludges from a further central Plymouth works, Camels Head, where there is limited room to handle the sludges on the compact line.
Oscar Faber was commissioned to provide design and project management services, using the principles of partnering with Civil and Meica contractors. The Ernesettle scheme was the fifth such works that Oscar Faber has been responsible for – two other schemes are currently under construction.
Camels Head sludges are digested, tankered to Ernesettle, stored, balanced and pressed (up to 25 per cent dry solids) to a cake for land usage or stored when adverse weather prevails. Ernesettle WwTW short term option is for primary sludges to be screened, digested and dealt with as the Camels Head sludges. The long term option involves the digesters being decommissioned in early 1999, with raw primary sludges stored, balanced and pressed, and passed on for further treatment utilising specially sealed skips.
The main scheme features two separate fully automated treatment lines, one for raw sludges and a second for imported digested sludges with the facility for cross connection from one stream to the other.
The new sludge works is to avoid encroaching upon likely areas for the construction of future secondary treatment plant while also not affecting existing processes. All odours are to be collected and treated to reduce offensive smells by a minimum of 99.5 per cent. All the liquors produced are to be collected, balanced and returned for treatment at a controlled rate and time.
The Camels Head imported digested sludge stream comprises two 650m3 storage tanks complete with submersible mixers, decants, roofs, access platforms and odour control facilities. The sludges are transferred to an underground sludge balancing sump, the belt press draws sludge from this for pressing to a cake, with the resultant cake being pumped into skips or trailers for use on land by local farmers.
Emergency sludge cake storage for 28 days production is provided in a purpose built agricultural type building for periods of adverse weather conditions and when disposal to land is not possible.
The presses liquors, sump overflows, decant liquors and all paved area runoff are returned via sealed and odour controlled drainage to an underground pumping station and on to an odour controlled liquor balancing tank for controlled discharge back to the works.
The Ernesettle short term option comprises one 650m3 sludge storage tank with submersible mixer, decants, roof, access platforms and odour control facility. Raw sludges are pumped using the existing ram pumps to a new Rotamat screen, where the sludges are screened and pumped back to the digesters for digestion. The sludges are passed by gravity from the digested sludge holding tanks to the new sludge storage tank. The sludges are again transferred automatically via a failsafe valve to an underground balancing sump. A new belt press processes the digested sludges and the dry cake is pumped into skips or trailers for land disposal.
The longer term option involves screening and pressing and then transporting to the new thermal drying centre currently being constructed as part of the Plymouth Central Works.
The change in emphasis, following a revision of South West Water’s sludge strategy and a more common acceptance of the principles of partnering, from a standard design and ICE 6th contract package to the use of partnering and the IChemE conditions of contract, has produced a scheme where all parties have been involved and had input to the design and construction phases throughout the project.